1. Field of the Invention
The present invention relates to a method for controlling lighting of a discharge lamp, and more particularly, to a method for controlling lighting of a discharge lamp with which constant current control is executed before constant power control. The present invention further relates to a computer program, an apparatus for controlling lighting of a discharge lamp, and a power supply circuit.
2. Description of the Related Art
Discharge lamps are categorized either as low-pressure discharge lamps or high-pressure discharge lamps depending on the pressure of their discharge gases. High-pressure discharge lamps may be further categorized as xenon lamps, high-pressure mercury lamps, or metal halide lamps. A metal halide lamp is a high-pressure discharge lamp in which metal halides are added to an arc discharge formed in high-pressure mercury vapor.
In a stable lighting status, discharge lamps are normally controlled through constant power control, with which the lamps consume less power. However, during a few-minute period after insulation breakdown, high-pressure discharge lamps, such as metal halide lamps, have a low lighting voltage, and therefore are required to be controlled through constant current control. More specifically, the high-pressure discharge lamps are controlled first through constant current control, and then are shifted to constant power control when their voltage reaches a predetermined value.
During the period after insulation breakdown before shifting to the constant power control, the high-pressure discharge lamps may overshoot or undershoot. Overshooting is the phenomenon in which a large current flows through a discharge lamp immediately after insulation breakdown. Undershooting is the phenomenon in which a small current flows through the discharge lamp as a reaction to overshooting. The discharge lamp may overshoot due to a delay in its control circuit or due to a capacitance of its smoothing circuit. When the discharge lamp overshoots, an excessively large current may flow through the lamp. This will damage the electrodes of the lamp. When the discharge lamp undershoots, the voltage of the lamp may decrease below a voltage at which an arc discharge can be maintained. This will turn off the lamp.
To prevent the discharge lamp from overshooting and undershooting, the reference current value, which serves as a target current of the lamp, may be increased in stages until the current of the lamp reaches a value at which the steady lighting is started (see, for example, Japanese Unexamined Patent Publication No. H7-176391).